Polycarbonate (PC) possesses many prominent properties such as impact resistance, heat resistance and high transmittance, etc., which is the second largest engineering plastic only next to nylon and PC has a wide variety of applications in the national economy. Due to the low melt tension, the conventional linear PC may lead to the phenomena such as the uneven thickness of the finished products and the contraction and the like during the process of its blow moulding, extrusion moulding and vacuo moulding. However, the branched PC can basically maintain the advantages of the linear PC and has better flowability, which is more suitable to be used in the situation requiring fine flowability to the polymer melt under higher rate of shear. Usually, adding one or more compound(s), which contain(s) three or more phenolic hydroxyl groups or acyl chloride groups, into the process of reaction can make the PC products have branched chain(s). The preparation process may be further divided into phosgenation interfacial method, melting transesterification method and reactive extrusion method, etc., wherein the most commonly used synthetic method is the phosgenation interfacial method.
Because of being affected by the cost and the stability in storage of raw materials, the polyphenolic compounds are preferred to be used as the branching agent in the industrialized production. According to the different process of adding branching agents, the conventional phosgenation interfacial method for preparing branched PC can be further divided into two kinds of processing routes, that is, one-step method and two-step method. One-step method is to prepare an aqueous solution of alkali metal hydroxide with the branching agent as well as bisphenol and then to react with phosgene. Two-step method is to react an alkali-metal-hydroxide aqueous solution of bisphenol with phosgene to prepare the oligomer firstly and then to add the branching agent to prepare high molecular weight branched PC. In the one-step method, obviously, there is a competing reaction between bisphenol/acyl chloride and branching agent/acyl chloride. In the two-step method, a large part of bisphenol has reacted with phosgene to form oligomer in the first step, but during the practical production, in order to control the molecular weight of the final product and ensure no residue of acyl chloride terminal group, it usually needs to control the conversion ratio of bisphenol below 99%, normally at 98˜99%, while the addition of branching agent is only 0.1˜2% (mol) of bisphenol, thus the competing reaction of bisphenol and branching agent with acyl chloride still cannot be avoided. Further, when under the same basic environment, the totally ionizing capability of branching agent is generally weaker than that of bisphenol, which results in bisphenol reacting with acyl chloride first and makes the conversion ratio of branching agent lower. In general, in order to improve the product's degree of branching and the conversion ratio, it is required to improve the addition amount of branching agent or the excessive ratio of phosgene, which may not only improve the cost of production, but also have a risk of conducting a crosslinking reaction and then reduce the quality of products.
WO9114728 discloses a technique of one-step method for feeding branching agent and bisphenol to prepare branched PC. In this technique, an aqueous phase of sodium phenolate is prepared with the branching agent THPE (1,1,1-tris(4-hydroxyphenyl)ethane) and BPA (bisphenol A), which is then reacted with phosgene in the existence of dichloromethane to produce branched polycarbonate oligomer, and at last the chain extension and end capping reactions are conducted to obtain the high molecular weight branched polycarbonate products. In this process, the conversion ratio of branching agent is relatively low, thus a higher addition of branching agent are required in order to obtain PC product with a high degree of branching, wherein the addition amount of THPE reaches 0.54 wt % of the total mass of BPA.
U.S. Pat. No. 5,104,964 provides a technique of one-step method for feeding branching agent and bisphenol to prepare branched PC. This process also makes the branching agent together with BPA form an aqueous phase of sodium phenolate, which is then reacted with phosgene in the existence of dichloromethane to produce oligomer emulsion. The aqueous phase is removed and then the oil phase of oligomer is obtained. The rest of dichloromethane, BPA, NaOH, PTBP (p-tert-butyl phenol) and TEA (triethylamine) are further added into the oligomer to conduct the chain extension and end capping reactions, and then the branched PC with high molecular weight is obtained. This process needs oil-water separation when preparing the oligomers, thereby increasing the reaction steps. Moreover, the interfacial tension of the oligomer emulsion is quite small, thus the practical process of separation is complicated.
US2009326149 also provides a technique of one-step method for feeding branching agent and bisphenol to prepare branched PC. In this technique, firstly, an oil-water two phases solution is prepared with BPA, dichloromethane, water, catalyst and NaOH etc., then a certain amount of THPE/NaOH solution is added into the oil-water two phases solution. Further phosgene is introduced step-by-step to prepare branched PC products. The pH value should be controlled at 8˜10 in this process. Though this technique is simple, the conversion ratio of the branching agent is rather low and there is residue of acyl chloride terminal group at about 20 ppm, which critically affects the quality of the products.
US2005020804 discloses a method for preparing branched PC by optimizing the process of adding catalysts. In this technique, the branching agent is also added into sodium salt aqueous solution of the BPA to participate in the reaction. This method applies two-step process to feed catalyst, wherein the first coupling catalyst is specially controlled to be added when the molecular weight (Mw) of oligomer is 1400-1600, and the second coupling catalyst is added when the Mw is increased to 10000.
U.S. Pat. No. 5,243,018 provides a technique of preparing branched PC by two-step feeding the branching agent and bisphenol. In this preparation process, BPA and phosgene and the like are used to synthesize the oligomer first, then the branching agent, catalyst and end capping agent, etc. are added into the oligomer to carry out the copolymerization reaction for preparing branched PC. In order to ensure the higher degree of branching and the property of products, many kinds of branching agents are adopted in this process.
CN1279698 provides a technique for preparing branched PC by using tetra-atomic phenol as the branching agent. In this method, tetra-atomic phenol are firstly reacted with BPA, phosgene etc., then a certain amount of BPA is added to react in the existence of catalyst TEA for preparing the oligomer, finally the rest of BPA, TEA and end capping agent and the like are added to react by the chain extension and end capping reaction to obtain the high molecular weight branched PC. In this process, BPA is added in three steps, and the catalyst is added in two steps, thus the process of reaction is very complicated. Moreover, too early adding the catalyst may have the risk of conducting the crosslinking reaction.
Based on the above illustration, in the present techniques by interface phosgene methods for preparing branched PC, it usually adopts reacting the branching agent as well as bisphenol with phosgene at the same time or reacting the bisphenol with phosgene to prepare oligomer and then reacting with the branching agent. All similar processes can produce branched PC, but because of the competing reaction between branching agent/acyl chloride and bisphenol/acyl chloride and the characteristics (such as the totally ionizing capability of branching agent is weaker than that of bisphenol), the conversion ratio of branching agent becomes lower. In the prior art, there is still no method for preparing branched polycarbonate by using the branching agent to react with a certain amount of phosgene for preparing the structure of poly-chloroformate firstly, then to react with bisphenol and the rest of phosgene and the like.